The warlock cloak is a holiday reward from the 2008 Hallowe'en event. It is a cloak with no bonuses, and is part of the warlock costume, which also includes the warlock top and the warlock legs.
It is no longer possible to obtain a warlock cloak, or any other part of the warlock costume. During the 2008 Hallowe'en event, players had to go trick-or-treating in Rimmington, Port Sarim, and Draynor Village to receive candy from certain NPCs. For this part of the costume, four chocolate drops needed to be received. Maggie would give you the cloak when you had all the chocolate drops.
Female characters cannot wear the warlock cloak, or any other part of the warlock costume. Female characters would receive a witch costume, and if they try to put a warlock cloak on (if they changed their gender), the cloak will turn into a witch cloak. If a male character tries to wear a warlock cloak while changing gender, it will also turn into a witch cloak.
In the members-only costume room of a player-owned house, the warlock costume can be stored in a toy box. If lost, the warlock costume can be re-obtained from Diango in Draynor Village.